yanmar diesel overheating

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joerockmore

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my 3GM30 is only 2 years old, but has recently been overheating. it starts up fine, but after running at normal cruising speeds (about 2700rpm) the temp goes from 160 to over 200 and the warning buzzer comes on. i have checked the raw sea water intake--no restriction. i checked the impeller on the raw water side pump--seems fine. the heat exchanger is not plugged, as i can blow in the hose that is the output of the pump and it is free to the exhaust pipe. on the fresh water side, the impeller in the pump seems fine, all hoses are clear, and water in the heat exchanger does in fact heat up. i also checked the thermostat and it opens fine in a pot of heated water. help!! any ideas what i can check now? any ideas on how i can get it fixed? thx in advance.
 
Did you measure the exact temp the stat opened, check with a thermometer, the opening temp is stamped on the stat.
 
Is it actualy overheating or is it just the sensor at fault, try removing the sensor and puting it in boiling water to see what it reads,
 
Brother-in-law had exaclty same problem. You need to take off the exhaust elbow and clean thouroghly.

This causes back-pressure and restricted water through the whole system when engine is running. May not be obvious when testing while engine is static.

Donald
 
It seems likely that the buzzer and the temp guage run off the same sensor. So the first port of call is to check that it is actually overheating, particularly since you have looked at the most obvious things that could cause the engine to overheat.
 
Joe - could you provide a few more details. Is there a good flow of water coming out the exhaust? How long does it take for the engine to o'heat? Did you physically check that the impellors are good, or just a visual? - as someone noted, the impellor can spin the bush, and no longer pump water.
 
I had an overheating problem on an Yanmar 2GM20, which was due to an airlock on the freshwater side after refilling with new antifreeze mix. It ran fine before launch, but the overtemp alarm came on 5 minutes after motoring away.
There was still an airspace in the engine.
David.
 
The sensor is easy to get out, take the wire off then use a good fitting socket or ring spanner (will most likey need a firm knock) to get it out. Cooling water will piss out everywhere shove a softwood plug in (gently!) turn on the ingition, turn off the beep if u can! and just put the sensor in boiled water and see if it goes off, then put it in water around the running temp. If it goes off then its knackerd, though thats a rare problem! Put the sensor back in loosely run it up, when the alarm goes off, gauge the temp of the water, in ours i am able to stick my finger in and then feel all over the head it should be warm but not red hot to the touch.. If that aint solved it then try this >>>>>>>>>>>>

Often the raw water side of the engine can be coated in Carbonate's in particular Calcuium Carbonate, this problem compounds it self when the engine starts to run at a high temp it precipitates out of seawater quciker. It can reduce the cooling by a lot! Get some de-scaler the type which does not attack metal and put it through the raw water side (remeber to leave it in long enough to do its magic) - I filled ours backwards using a funnel from on the pipe that goes into the mixing elbow so it simply filled the raw water side and it will come evenutally out of by the pump. You get some indication something happening if it starts to bubble in the funnel. If that dont work take the mixing elbow off, the gasket is only £5, and its only three bolts, check to see its not caked up, if it is knock all the coke out of it... Then last resort (as it messes up the paint on the front of the engine) take the stat out and boil it, it should open well before the boil.

If you are lossing coolant either out of the cap or aparently to no where then your head gasket's gone. Otherwise don't touch the head!

If your on a saildrive the bronze right angle tap on the side of the SD unit often calcites up on the exposed threads, but theres not much you can do to take it out, but if ur brave you can whip it out and shove a soft wood plug rapped in soft ducktape in the whole which u give the seacock a bath in the afore mentioned de-scale.

Remeber you wont notice any resitance when you blow through the pipes! Its does not indicate the passages are clear!

...
 
With the 2GM20 at least a common cause of this is very simple: if the top of the water filter doesn't have a good seal! eg. if you opened it to remove some seaweed and didn't screw it back on well enough to create a seal. If a vacuum doesn't form in the filter tub, then the impeller sucks air instead of water.
 
wow! thx for all the great responses. i'll answer the questions posed, but you all gave me more to think about and try. there is indeed water spitting out the exhaust, so there is at least flow through the raw water side (i can't tell if it is sufficient, however). i did not measure the exact temp the thermostat opened (don't carry a thermometer on the boat), but was going to replace it anyway. no oil loss recently. as for the sensor, there are two, one for the guage and one for the buzzer, and both indicate overheat, so i don't think its that. as for how long it takes to overheat, when running at cruising rpm (about 2700) it takes about 10 minutes for the temp to rise sufficiently for the buzzer to go off (about 190-200 degrees). as for inspection of the impellers, i took each apart and visually inspected, but also spun them to ensure they are tight and that the vanes have a good seal with the walls. what else should i do to check them? again, thx for all the suggestions.
 
thx for all your replies and suggestions. it turns out that what was wrong was the exhaust pipe out of the engine that is an inverted U shape that takes the raw water output was clogged. i had checked it up to that point, but when i removed the U and tried to push water through it with a hose, i only got a trickle. cleaning that out with a piece of stiff wire (plenty of gunk came out) did the trick. a lesson for us all: clean the exhaust out once in a while. if the tube inside the exhaust pipe sprang a leak, it would back sea water into the exhaust manifold, probably hurting the engine. live and learn...
 
thanks for suggestions on yanmar diesal overheating

this was very helpful in solving our problem. turned out we needed to clean the exhaust elbow! lots of time invested but this saved us lots of aggravation and money! thanks for all the dialog!
 
I'd still double-check the impeller. The brass may be slipping inside the rubber, which will drive some water through at idle and low revs, but not enough, or any, under load. I've had that a couple of times with my 1GM10.
 
Are you sure that it is a 3GM30 as they were superseded by the 3YM30 about 5 years ago. Early 3YM30’s are infamous for overheating problems.
 
joerockmore said:
it turns out that what was wrong was the exhaust pipe out of the engine that is an inverted U shape that takes the raw water output was clogged. i had checked it up to that point, but when i removed the U and tried to push water through it with a hose, i only got a trickle. cleaning that out with a piece of stiff wire (plenty of gunk came out) did the trick. a lesson for us all: clean the exhaust out once in a while.
Click to expand...

What was blocking it? sooty gunk or bits of the pipe? When they get old, exhaust hoses can delaminate internally.
 
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